Note that you can also use negative values on the channels. For example:
Channel
% Value
Additional Value
Red
-100%
256
Green
-100%
256
Blue
-100%
256
Alpha
100%
0
This will invert all the channels - transforming its "brightness values" into, in fact, "darkness values" - and then pumping its values (that are now negative) up again by adding a 256
value to them (the maximum value a color channel can have). The result is an "inverse" image: